In drilling oil and gas wells or mineral mines, earth-boring drill bits are commonly used. Typically, an earth-boring drill bit is mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface. With weight applied to the drill string, the rotating drill bit engages an earthen formation and proceeds to form a borehole along a predetermined path toward a target zone.
A rock bit, typically used in drilling oil and gas wells, generally includes one or more rotatable cones (also referred as to "rolling cones") that perform their cutting function through the rolling and sliding movement of the cones acting against the formation. The cones roll and slide upon the bottom of the borehole as the bit is rotated, thereby engaging and disintegrating the formation material in its path. A borehole is formed as the gouging and scraping or crushing and chipping action of the rolling cones removes chips of formation material that are then carried upward and out of the borehole by circulation of a liquid drilling fluid or air through the borehole. Petroleum bits typically use a liquid drilling fluid which is pumped downwardly through the drill pipe and out of the bit. As the drilling fluid flows up out of the borehole, the chips and cuttings are carried along in a slurry. Mining bits typically do not employ a liquid drilling fluid; rather, air is used to remove chips and cuttings.
The earth-disintegrating action of the rolling cone cutters is enhanced by a plurality of cutter elements. Cutter elements are generally inserts formed of a very hard material which are press-fit into undersized apertures or sockets in the cone surface. Due to their toughness and high wear resistance, inserts formed of tungsten carbide dispersed in a cobalt binder have been used successfully in rock-drilling and earth-cutting applications.
Breakage or wear of the tungsten carbide inserts limits the lifetime of a drill bit. The tungsten carbide inserts of a rock bit are subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impacting loads from contact with the borehole bottom. Also, the high wear load can cause thermal fatigue in the tungsten carbide inserts which can initiate surface cracks on the inserts. These cracks are further propagated by a mechanical fatigue mechanism caused by the cyclical bending stresses and/or impact loads applied to the inserts. This may result in chipping, breakage, and/or failure of inserts.
Inserts that cut the comer of a borehole bottom are subject to the greatest amount of thermal fatigue. Thermal fatigue is caused by heat generation on the insert from a heavy frictional loading component produced as the insert engages the borehole wall and slides into the bottom-most crushing position. When the insert retracts from the borehole wall and the bottom of the borehole, it is quickly cooled by the circulating drilling fluid. This repetitive heating and cooling cycle can initiate cracking on the outer surface of the insert. These cracks are then propagated through the body of the insert when the crest of the insert contacts the borehole bottom, as high stresses are developed. The time required to progress from heat checking to chipping, and eventually, to breaking inserts depends upon formation type, rotation speed, and applied weight.
Thermal fatigue is more severe in mining bits because more weight is applied to the bit and the formation usually is harder, although the drilling speed is lower and air is used to remove cuttings and chips. In the case of petroleum bits, thermal fatigue also is of serious concern because the drilling speed is faster and liquid drilling fluids typically are used.
Cemented tungsten carbide generally refers to tungsten carbide ("WC") particles dispersed in a binder metal matrix, such as iron, nickel, or cobalt. Tungsten carbide in a cobalt matrix is the most common form of cemented tungsten carbide, which is further classified by grades based on the grain size of WC and the cobalt content.
Tungsten carbide grades are primarily made in consideration of two factors that influence the lifetime of a tungsten carbide insert: wear resistance and toughness. As a result, existing inserts are generally formed of cemented tungsten carbide particles (with grain sizes in the range of about 3 .mu.m to 6 .mu.m) and cobalt (the cobalt content in the range of about 9% to 16% by weight. However, thermal fatigue and heat checking in tungsten carbide inserts are issues that have not been adequately resolved. Consequently, inserts made of these tungsten carbide grades frequently fail due to heat checking and thermal fatigue when high rotational speeds and high weights are applied.
For the foregoing reasons, there exists a need for a new cemented tungsten carbide grade with the desired toughness, wear resistance, and improved thermal fatigue and shock resistance so that better inserts may be manufactured from the new grade, and better drilling bits may be made using these inserts.